A through-substrate via (TSV) is a vertical electrical connection passing completely through a substrate that has integrated circuitry therein. TSVs may be used to create 3D integrated circuits packages, and are an improvement over other techniques such as package-on-package because the density of through-substrate vias may be substantially higher. TSVs provide interconnection of vertically aligned electronic devices through internal wiring that may reduce complexity and overall dimensions of a multi-chip integrated circuit.
Some individual integrated circuit substrates containing TSVs have a bond pad connected adjacent one end of the TSV on one side of the substrate. A pillar-like conductive structure is connected adjacent the other end and projects from the other side of the substrate, with the elevationally outermost portion thereof being solder. Two integrated circuit substrates may be bonded together by placing that solder against aligned bond pads of another substrate. The resultant construction can then be heated to cause the solder to flow and bond with the respective bond pads. Solder flux may be applied to the solder prior to bringing the substrates into contact with one another. The solder flux contains tackiness agents which facilitate holding the immediately adjacent substrates together until sufficient heating can occur to bond the solder with the bond pads. Once the bonding is complete, a dielectric underfill material may be provided between the substrates for added support and protection.
Residual solder flux may be removed by cleaning prior to flowing the dielectric underfill material between the substrates. Alternately, some solder fluxes are referred to in the industry as “no clean” whereby solder flux residue intentionally remains between the substrates prior to flowing the dielectric underfill material. Regardless, even with attempted cleaning away of solder flux residue prior to dielectric underfill, not all of the residue normally gets removed. Difficulty in removing solder flux in such instances has increased as the separation spacing between immediately adjacent circuit substrates has become smaller.
The construction with dielectric underfill material may be subjected to subsequent heating which can cause gasification of solder flux residue. This can lead to structural failures including separation of the solder bonds from the bond pads. This has become particularly problematic as the spacing between immediately adjacent substrates continues to reduce, specifically at distances of 40 microns or less.